We propose to study a novel, tissue specific control mechanism, by which the product specificity of an enzyme is modified by a second factor, probably a protein. In the lactating mammary gland, the product specificity of the fatty acid synthetase is modified by a factor which shifts the product chain length from long chain to the medium chain fatty acids characteristic of milk fat. The objectives of the proposal are to identify and isolate the chain length modifier, to study its physicochemical properties, the mechanism of its interaction and the fatty acid synthetase and the hormonal regulation of its biosynthesis.